WRKY protein domain
solution structure of the c-terminal wrky domain of atwrky4
In molecular biology, WRKY transcription factors are a class of sequence-specific DNA-binding transcription factors found almost exclusively in plants. These transcription factors, have diverse biological functions due to their ability in regulating expression of a wide range of target genes through coordination with other DNA-binding or non-DNA-binding interacting proteins. WRKY proteins are thought to play an important role in plant defense responses, plant hormone signaling, secondary metabolism and plant responses to abiotic stress.
WRKY transcription factors
The first two reports of WRKY transcription factors appeared in 1994-95 and were of ill-defined DNA binding proteins that played potential roles in the regulation of gene expression by sucrose (SPF1) or during germination (ABF1 and ABF2). These were followed by the identification of WRKY1, WRKY2 and WRKY3 from parsley (Petroselinum crispum) and the name WRKY (pronounced ‘worky’) was coined. This report also provided the first evidence that WRKY proteins play roles in regulating plant responses to pathogens, and many reports have since shown this to be a major role of WRKY transcription factors. Increasingly, however, WRKY proteins are being shown to regulate other processes such as abiotic stress responses, seed germination, senescence and wound responses and it is becoming clear that a single WRKY transcription factor might be involved in regulating several seemingly disparate plant processes.
The WRKY domain is a 60 amino acid region that is defined by the conserved amino acid sequence WRKYGQK at its N-terminal domain, together with a novel zinc-finger-like motif. The WRKY domain is found in one or two copies in a superfamily of plant transcription factors involved in the regulation of various physiological programs that are unique to plants, including pathogen defence, senescence, trichome development and the biosynthesis of secondary metabolites. The WRKY domain binds specifically to the DNA sequence motif (T)(T)TGAC(C/T), which is known as the W box. The invariant TGAC core of the W box is essential for function and WRKY binding. Some proteins known to contain a WRKY domain include Arabidopsis thaliana ZAP1 (Zinc-dependent Activator Protein-1) and AtWRKY44/TTG2, a protein involved in trichome development and anthocyanin pigmentation; and wild oat ABF1-2, two proteins involved in the gibberelic acid-induced expression of the alpha-Amy2 gene.
Structural studies indicate that this domain is a four-stranded beta-sheet with a zinc binding pocket, forming a novel zinc and DNA binding structure. The WRKYGQK residues correspond to the most N-terminal beta-strand, which enables extensive hydrophobic interactions, contributing to the structural stability of the beta-sheet.
From the beginning of research into WRKY transcription factors, it was evident that they play roles in regulating several different plant processes. Recent data has led to a new insight in this area, namely that it is common for a single WRKY transcription factor to regulate transcriptional reprogramming associated with multiple plant programs. The dynamic web of signaling in which WRKY factors operate has multiple inputs and outputs.
Mechanisms of action
WRKY proteins can activate or repress transcription and substantial evidence now indicates that many genes are repressed by WRKY factors bound to their promoters. The WRKY proteins themselves appear to function via numerous different interacting partners including a diverse array of protein partners, including MAP kinases, MAP kinase kinases, 14-3-3 proteins, calmodulin, histone deacetylases, resistance proteins and other WRKY transcription factors. WRKY genes also exhibit extensive autoregulation and cross-regulation that facilitates transcriptional reprogramming in a dynamic web with built in redundancy.
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